The present invention relates to a system for error correction which is suitable for correcting either machine errors or division errors in precision measuring instruments of the type used to measure the relative position of first and second objects. In particular, this invention is suited for use in precision measuring instruments of the type comprising a measuring scale which defines a measuring division, a scanning unit adapted to scan the division and to generate at least one analog measurement signal in response thereto, means for coupling the scale to the first object, means for coupling the scanning unit to the second object, and an evaluating unit coupled to the scanning unit to evaluate the at least one analog measurement signal.
A variety of error correction systems are known to the art for use with position measuring instruments. In German Pat. No. 27 24 858 a correction system for a length measuring device is disclosed which includes a link chain, the position of which is sensed by means of a transfer element. The lengths of this link chain are adjustable transversely to the measurement direction according to the magnitude of the error to be corrected. The accuracy and adjustability of this error correction system depends on the number of links used per unit of length in the measuring direction.
German Pat. No. 28 20 753 discloses an error correction system for position measuring instruments in which an error correction profile is formed as an integral component of a housing for a scale. This error correction profile as sensed by means of a transfer element which brings about a correcting relative movement between the scanning unit of the measuring system and the scale.
German DE-OS No. 27 29 697 discloses a process for interpolating periodic analog signals of a digital electric position measuring instrument. In the disclosed process, the analog signals are first digitized and the digital values are applied as inputs to a computer which generates the interpolation values. Particularly in applications in which a high subdivision factor is used in interpolation, corrections are made to the digital value stored in the computer prior to interpolation, which corrections compensate for faulty signal parameters of the analog signals. Previously determined correction values are stored in computer memory for later use.
In the correction systems described in the two patents identified above, additional mechanical elements are required to carry out the correction, and these additional mechanical elements can result in substantially increased external dimensions of the position measuring instrument. Such increases in external dimensions can be a severe drawback, and can make the measuring system unsuitable for certain applications. The position measuring system described in DE-OS No. 27 29 697 requires complicated electronic components to carry out the corrections.